The present invention relates to a new circuit and method for controlling the driving of a load, and more specifically, a driving circuit, controlled by a pulse width modulated signal, that is capable of driving a load at a maximum power level.
A variety of today""s electrical systems rely on pulse width modulation (PWM) of a signal to control analog circuits and devices in a digital manner. According to basic PWM techniques, a voltage or current source is supplied to an analog load, such as a motor, by means of a repeating series of on or off pulses. The power supply is fully on and applied to a load only during the on-times defined by the repeating series of on and off pulses. The subsequent ratio of on-time to period of the signal is known as the duty cycle of a PWM signal and is expressed in percentages. Thus, a PWM signal with a 50% duty cycle represents a signal comprised of on pulses for half of the time, while a 100% duty cycle represents the power supply being continuously applied to a load.
PWM signal control is often utilized with bootstrap-type driving circuits, which rely on the use of a first power supply to activate or turn xe2x80x9conxe2x80x9d a circuit that subsequently drives a load using a second power supply. FIG. 1 illustrates the general layout of a known bootstrap-type driving circuit 100 that utilizes a pulse width modulated control signal. In general, the purpose of circuit 100 is to drive load 130 using a primary power supply Vp. This is carried out by means of switch 120. When switch 120 is placed in an xe2x80x9conxe2x80x9d state, electrical current to flows from the primary power supply Vp, through the switch 120, to the load 130, and when switch 120 is xe2x80x9coffxe2x80x9d, no current flows from primary power supply Vp to the load.
Controlling the xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d state of primary switch 120 is a secondary switch 140 that xe2x80x9cflipsxe2x80x9d between a first and second state, thereby connecting either a first path (A) or second path (B) to capacitance 150, depending on a PWM control signal Vin. Specifically, when the control signal Vin is off/low, switch 140 is placed in a first state whereby capacitance 150 is connected to an auxiliary power supply Va, such as, for example, a 12 Volt source, through first circuit path (A). Accordingly, when control signal Vin is off/low, switch 120 remains in its default xe2x80x9coffxe2x80x9d state. At the same time, capacitance 150 is charged by electrical current that is permitted to flow from the auxiliary power supply Va, to the capacitance 150, and then through the load 130.
When control signal Vin is on/high, secondary switch 140 is placed in a second state whereby capacitance 150 is connected to primary switch 120 by means of the second path (B). This results in primary switch 120 turning xe2x80x9conxe2x80x9d due to application of the built-up charge stored in capacitance 150. Consequently, with switch 120 xe2x80x9conxe2x80x9d, the primary power supply Vp is able to drive load 130.
Accordingly, when the PWM control signal Vin, applied to secondary switch 140, is off/low, primary switch 120 remains off while capacitance 150 is charged. Conversely, when control signal Vin is high, the built-up charge on capacitance 150 is applied to primary switch 120, thereby placing switch 120 in an xe2x80x9conxe2x80x9d state and allowing the primary power supply Vp to drive load 130 until the PWM control signal Vin goes off/low again.
The bootstrap-type driving circuit described above works sufficiently for driving a load 130 at less than maximum power levels, such as, for example, upon application of a control signal Vin having less than a 100% duty cycle. However, complications arise when one attempts to fully drive load 130 at a maximum power level. This is because bootstrap-type driving circuits utilizing PWM control, as generally described above, are unable to function properly upon the application of a PWM control signal Vin having a sufficiently high duty cycle. The reason for this is because at sufficiently high duty cycle levels, such as, for example, a 100% duty cycle, PWM signals are effectively converted from a series of on and off pulses to a constant voltage or current signal. Application of an essentially constant control signal Vin to circuit 100 above results in secondary switch 140 being placed in its secondary state. Furthermore, secondary switch 140 will remain in its secondary state for as long as the essentially constant control signal Vin is applied. During this time period, capacitance 150 is connected to primary switch 120, with the charge on capacitance 150 placing switch 120 in an xe2x80x9conxe2x80x9d state. However, capacitance 150, like all capacitances, is subject to a condition known as xe2x80x9cvoltage droopxe2x80x9d, whereby, in the absence of periodic recharging, which normally occurs at lower duty cycles, the stored charge on capacitance 150 quickly diminishes due to current leakage. Consider, for example, the situation where a control signal having a 100% duty cycle is applied to the circuit. Unless capacitance 150 is periodically recharged, the stored charge on capacitance 150 may only last a few milliseconds before being reduced to an insufficient voltage amount. Yet, because of the high duty cycle of the control signal, capacitance 150 will not be provided with a chance to recharge. As a result, after those few milliseconds, the charge stored on capacitance 150 is no longer sufficient to maintain the primary switch 120 in an xe2x80x9conxe2x80x9d state.
Accordingly, the application of a PWM input signal Vin having a sufficiently high enough duty cycle results in a lack of periodic recharging of capacitance 150. Consequently, without periodic recharging of capacitance 150, voltage droop becomes a significant factor, leading to capacitance 150 having an insufficient charge to maintain primary switch 120 in an xe2x80x9conxe2x80x9d state. As a result, the inventor of the present invention has realized the need for a bootstrap-type driving circuit that utilizes a pulse width modulated (PWM) control signal to control the variable driving of a load, including driving the load at or near a maximum power level upon the application of a sufficiently high enough PWM control signal.
The present invention relates to a circuit and method for electrically driving a load. The circuit includes the use of a driving circuit for variably driving the load in response to a pulse width modulated control signal. Also included is a compensation circuit that permits the driving circuit to drive the load at a maximum power level when the pulse width modulation control signal has a sufficiently high enough duty cycle.